There are a few that I know of:
First, fast traveling to one specific spot like someone else said. Very effective for some areas, and for others not so much.
The bench method is good. I did that so much in the ice area (zone 12 I think) and got way to many shinies.
Having one run back n forth. Like, into the wild zone, forward say, five steps, turn, walk back out the wild zone. Also could work for fast traveling. Like, fast travel, go into the wild zone, walk a few steps fast travel again.
For specifically Gastly, I don't know. I'm assuming going to wild zone 4 (I think that's the graveyard one) and fast traveling would work.
Sorry if this isn't super accurate.